Low-emissivity coatings are well known in the art. Typically, they include one or more layers of infrared-reflective material and two or more transparent dielectric layers. The infrared-reflective layers, which are typically conductive metals such as silver, gold, or copper, reduce the transmission of radiant heat through the coating. The transparent dielectric layers are used primarily to reduce visible reflectance and to control other properties of the coatings, such as color. Commonly used transparent dielectrics include oxides of zinc, tin, and titanium. Low-emissivity coatings are commonly deposited on glass sheets through the use of well known magnetron sputtering techniques.
It is often necessary to heat glass sheets to temperatures at or near the melting point of glass to temper the sheets or to enable them to be bent into desired shapes, such as motor vehicle windshields. Tempering is important for glass used in automobile windows, and particularly for glass used in automobile windshields. Upon breaking, tempered glass desirably exhibits a break pattern in which the glass shatters into a great many small pieces, rather than large dangerous shards. This coated glass must often be able to withstand elevated tempering temperatures, commonly on the order of about 600 degrees C. and above, for significant periods of time (e.g., hours).
Many low-emissivity film stacks that employ silver as the infrared-reflective material (i.e., silver-based low-emissivity coatings) cannot withstand elevated tempering temperatures without some deterioration of the silver. In one reported method for avoiding this problem, glass sheets are heated and bent or tempered before they are coated, and are later provided with the desired coatings. However, this procedure tends to be complicated and costly and, more problematically, may produce non-uniform coatings.
Another reported method for protecting reflective silver films from deterioration at high temperatures involves sandwiching each silver film between protective layers of an oxidizable and/or nitridable metal. The protective layers are thick enough and reactive enough that when the coated glass is heated to high temperatures, these layers capture oxygen and/or nitrogen that might otherwise reach and react with the reflective silver films. While these protective layers prevent oxygen and nitrogen from reaching the silver films, they become largely oxidized and/or nitrided themselves. Insofar as the oxides and nitrides of the protective metals are more transparent than the protective metals themselves, this typically causes an increase in the transmissivity of the coating. Reference is made to Huffer et al. U.S. Pat. No. 4,790,922, Finley U.S. Pat. No. 4,806,220, and Gillery U.S. Pat. No. 3,962,488, the entire teachings of each of which are incorporated herein by reference.
U.S. Pat. No. 5,344,718 (Hartig et al.) describes use of a film stack in which silver is sandwiched between layers of nickel or nichrome and the resulting sandwich is positioned between films of silicon nitride. It is said that when a Ni:Cr alloy is employed, the chromium during sputtering is converted at least in part to a nitride of chromium and that visible transmittance is thus improved.
U.S. Pat. Nos. 6,060,178 and 6,231,999, both issued to Krisko, disclose particularly useful heat-treatable coatings in which niobium is positioned on one or both sides of an infrared-reflective layer. When an oxide or nitride film is subsequently deposited over the niobium (e.g., when such film is sputtered onto the niobium in a reactive oxidizing or nitriding atmosphere), the niobium may be converted at least in part to an oxide or nitride of niobium. Insofar as the oxides and nitrides of niobium are more transparent than metallic niobium, this increases the visible transmittance of the coating. The entire teachings of each of these patents (the “Krisko niobium patents”) are incorporated herein by reference.
Protective layers should be deposited at sufficient thickness to prevent deterioration of infrared-reflective material (e.g., silver) during heat treatment. When these protective layers are too thin, they tend not to fully protect the infrared-reflective material during heat treatment. For example, when the protective layer or layers in a low-emissivity coating are too thin, the coating may develop a white hazy appearance (which is referred to herein as “white haze”) when tempered or otherwise heat-treated. The precise mechanism behind white haze formation does not appear to have been satisfactorily explained. However, it is surmised to be a result of oxygen reaching, and reacting with, the infrared-reflective material in the coating. For example, when silver in a low-emissivity coating is not aptly protected, tempering appears to cause the silver to become non-continuous, forming islands of silver breaking up the originally continuous layer. On the other hand, when protective layers are too thick, they tend not to be oxidized to the desired extent, leaving the coating more reflective and/or less transparent than is preferred. In the case of niobium protective layers, this can yield a coating with a somewhat reddish appearance, due to the color of metallic niobium. Thus, care should be taken to assure that protective layers of an appropriate thickness and reactive state are selected for heat-treatable coatings.
The application of heat to coated glass (e.g., during tempering and other heat treatments) tends to exacerbate the visible impact of any defects on the glass. For example, substrate defects may first appear, or may become more visible, after heat treatment. Therefore, substrate quality is particularly important in the production of heat-treatable coatings.
For example, it is preferable to deposit thin films on newly manufactured (i.e., fresh) glass. As is well known in the art, glass is vulnerable to becoming corroded when exposed to moisture (e.g., water in the air of ambient glass processing and storage environments). In fact, it is virtually impossible for a manufacturer of coated glass to assure that the raw glass it uses (i.e., coats) is completely free of moisture corrosion.
Moisture corroded glass typically has a rough surface. For example, corroded glass may exhibit different degrees of surface roughness in different areas. This can be appreciated in FIG. 1, wherein there is illustrated a substrate 10 having relatively rough 17 and smooth 14 surface areas. The rough areas 17 may comprise a series of small peaks and valleys. While FIG. 1 is not intended to represent precisely the surface roughness that characteristically results from moisture corrosion, moisture corroded glass has been found to exhibit locally varying degrees of roughness.
Surface defects may also result when uncoated glass sheets are engaged by glass handling equipment. For example, rollers and suction cups can leave surface modifications (e.g., increased roughness, scuffmarks, or other traces of contact) on the glass. These surface modifications can impact the coatings that are ultimately deposited on the uncoated glass. For example, traces of contact from suction cups and/or rollers may become visible, or more visible, when sheets of coated glass are tempered or otherwise heat-treated.
Thus, the manufacture of high quality coated glass can be challenging due to the commonly less than optimal quality of raw (i.e., uncoated) glass. For example, according to conventional wisdom in the art, when thin films are coated upon a rough (e.g., corroded) substrate surface, the roughness of the coating tends to increase as more and more film is deposited. Not only may this yield coated glass with an undesirably rough coated surface, it can also have adverse effects on the desired properties of the coating.
For example, temperable low-emissivity glass has been found to exhibit an objectionable appearance more frequently when produced with glass that is old (and more likely corroded by exposure to moisture) than when produced with fresh glass. Similarly, temperable low-emissivity glass has been found to exhibit an objectionable appearance more frequently when produced with glass that has been stored under conditions promoting moisture corrosion (e.g., high humidity). Such glass tends to have a non-uniform appearance, characterized by local areas of haze and low transparency.
In an attempt to obviate this problem, one might increase the thickness of the protective layer overlying each reflective silver film. However, this may not improve the appearance of the coated glass. For example, the protective layer may not be oxidized and/or nitrided as fully as is desired. As a consequence, the visible reflectance of the coating may be greater than is preferred. Further, the transparency of the coating may be decreased, and the color of the coated glass may be adversely affected. For example, increasing the thickness of niobium protective layers has been found to yield glass with local areas of reddish haze. Surprisingly, no matter what thickness is selected for the protective layers, there tend always to be local areas of haze (e.g., white haze, red haze, or both) and low transparency, which give the glass a non-uniform appearance. Glass of this nature would typically be rejected in the market place.
It would be desirable to provide thin film coatings that retain their integrity even when applied to corroded substrates. It would be particularly desirable to provide heat-treatable coatings (e.g., temperable low-emissivity coatings) that resist deterioration (e.g., haze formation) even when applied to corroded substrates (e.g., moisture corroded glass).